An organic electroluminescence (“electroluminescence” will be occasionally referred to as “EL”, hereinafter) device is a spontaneous light emitting device which utilizes the principle that a fluorescent substance emits light by energy of recombination of holes injected from an anode and electrons injected from a cathode when an electric field is applied. Since an organic EL device of the laminate type driven under a low electric voltage was reported by C. W. Tang et al. of Eastman Kodak Company (C. W. Tang and S. A. Vanslyke, Applied Physics Letters, Volume 51, Pages 913, 1987), many studies have been conducted on organic EL devices using organic materials as the constituting materials. Tang et al. used a laminate structure using tris(8-quinolinolato)aluminum for the light emitting layer and a triphenyldiamine derivative for the hole transporting layer. Advantages of the laminate structure are that the efficiency of hole injection into the light emitting layer can be increased, and that the efficiency of forming excitons which are formed by blocking and recombining electrons injected from the cathode can be increased, and that excitons formed among the light emitting layer can be enclosed. As the structure of the organic EL device, a two-layered structure having a hole transporting (injecting) layer and an electron transporting and light emitting layer or a three-layered structure having a hole transporting (injecting) layer, a light emitting layer and an electron transporting (injecting) layer are well known. To increase the efficiency of recombination of injected holes and electrons in the devices of the laminate type, the structure of the device and the process for producing the device have been studied.
As the light emitting material of the organic EL device, chelate complexes such as tris(8-quinolinolato)aluminum, coumarine derivatives, tetraphenylbutadiene derivatives, bisstyrylarylene derivatives and oxadiazole derivatives are known. It is reported that light in the visible region ranging from blue light to red light can be obtained by using these light emitting materials, and development of a device exhibiting color images is expected (refer to, for example, Patent literatures 1, 2, and 3).
In addition, it has been recently proposed that an organic phosphorescent material is applied to a light emitting layer besides a light emitting material (refer to, for example, non-Patent literatures 1 and 2). A high efficiency of light emission has been achieved by utilizing a singlet state and a triplet state of the organic phosphorescent materials in a light emitting layer of an organic EL device. Since it has been considered that singlet exciton and triplet exciton are formed at a ratio of 1 to 3 due to difference of spin multiplicity thereof on recombination of electrons and holes in an organic EL device, it should be understood that three to four times higher efficiency of light emission can be achieved by utilizing an phosphorescent light emitting material than by utilizing only a fluorescent light emitting material.
In the above organic EL devices, in order to avoid quenching of triplet excited state or excitons in triplet state, a construction comprised by laminating sequentially an anode, a hole transporting layer, an organic light emitting layer, an electron transporting layer (a hole blocking layer), an electron transporting layer, a cathode and so forth has been employed, and a host compound and a phosphorescent compound have been employed as an organic light emitting layer (refer to, for example, Patent literatures 4 and 5). The patent literatures relate to an organic phosphorescent material emitting a light in the range of from red to green. In addition, technologies relating to a material emitting a blue light are disclosed (refer to, for example, Patent literatures 6, 7 and 8). However, these have very short lifetimes Ligand-frameworks bonded with Ir metal and phosphorous atom are described in Patent literatures 7 and 8. However, although they emit blue light, the bond strengths are week so that the heat resistances are extremely poor. Additionally, a complex of which the central metal is bonded with an oxygen atom and a nitrogen atom, is described in Patent literature 9. However, there is no description of any specific advantage of the group bonding to oxygen atom. Further, the complex, of which the central metal bonded with each nitrogen atom being contained in different ring structures, is disclosed in Patent literature 10. Although a device using it exhibits a blue light emission, the external quantum efficiency is low as about 5%.
Patent literature 1: Japanese Patent Application Laid-open No. Heisei 8 (1996)-239655,
Patent literature 2: Japanese Patent Application Laid-open No. Heisei 7 (1995)-138561,
Patent literature 3: Japanese Patent Application Laid-open No. Heisei 3 (1991)-200889,
Patent literature 4: U.S. Pat. No. 6,097,147,
Patent literature 5: International PCT publication No. WO01/41512,
Patent literature 6: U.S. Patent Application Laid-open No. 2001/0025108,
Patent literature 7: U.S. Patent Application Laid-open No. 2002/0182441,
Patent literature 8: Japanese Patent Application Laid-open No. 2002-170684,
Patent literature 9: Japanese Patent Application Laid-open No. 2003-123982,
Patent literature 10: Japanese Patent Application Laid-open No. 2003-133074,
Non-patent literature 1: D. F. OBrien and M. A. Baldo et al “Improved energy transfer in electrophosphorescent devices” Vol. 74 No. 3, pp 442-444, Jan. 18, 1999, and
Non-patent literature 2: M. A. Baldo et al “Very high-efficiency green organic light-emitting devices based in electrophosphorescence” Applied Physics letters, Vol. 75 No. 1, pp 4-6, Jul. 5, 1999.